1. Field of the Invention
The present invention generally relates to drill bits useful for drilling oil, gas and water wells and methods for manufacturing such bits. More specifically, the present invention relates to a bi-center bit or two-piece bi-center bit which includes a sub-reamer section to aid in enhancing stability of the tool when rotated in the borehole.
2. Description of the Prior Art
A significant source of many drilling problems relates to drill bit and string instability. Bit and/or string instability probably occurs much more often than is readily apparent by reference to immediately noticeable problems. However, when such instability is severe, it places high stress on drilling equipment that includes not only drill bits but other downhole tools and the drill string in general. Common problems caused by such instability may include, but are not limited to, excessive torque, directional drilling control problems, and coring problems.
One typical approach to solving these problems is to over-design the drilling tool to thereby resist the stress. However, this solution is usually expensive and can actually limit performance. For instance, one presently commercially available drill bit includes reinforced polycrystalline diamond compact (“PDC”) members that are strengthened by use of a fairly large taper or frustoconical contour on the PDC member. The taper angle in this tool is smaller than the backrake angle of the cutter to allow the cutter to cut into the formation at a desired angle. While this design makes the PDC cutters stronger so as to reduce cutter damage, it does not solve the primary problem of bit instability. Thus, drill string problems, directional drilling control problems, and excessive torque problems remain. Also, because the PDC diamond table must be ground on all of the PDC cutters, the drill bits made in this manner are more expensive and less resistant to abrasive wear as compared to the same drill bit made with standard cutters.
Another prior art solution to bit instability problems is directed toward a specific type of bit instability that is generally referred to as bit whirl. Bit whirl is a very complicated process that includes many types of bit movement patterns or modes of motion wherein the bit typically does not remain centered within the borehole. The solution is based on the premise that it is impossible to design and build a perfectly balanced bit. Therefore, an intentionally imbalanced bit is provided in a manner that improves bit stability. One drawback to this method is that for it to perform properly in the borehole, the bit forces must be the dominant force acting on the bit. These bits are generally designed to provide for a cutting force imbalance that may range about 500 to 2000 pounds depending on bit size and type. Unfortunately, there are many cases where gravity or string movements create forces larger than the designed cutting force imbalance and therefore become the dominant bit forces. In such cases, the intentionally designed imbalance is ineffective to prevent the tool from becoming unstable and whirling in operation in the borehole.
Yet other attempts to reduce bit instability have incorporated devices that are generally referred to as penetration limiters. Penetration limiters work to prevent excessive cutter penetration into the formation that can lead to bit whirl or cutter damage. These devices may act to prevent not only bit whirl but also prevent radial bit movement or tilting problems that occur when drilling forces are not balanced. Furthermore, penetration limiters reduce bit rate of penetration of used too extensively but do not significantly improve stability of used too sparingly. Due to the variety of different applications, it is frequently difficult to determine to what extent penetration limiters should ve used.
While the above background has been directed to drill bits in general, more specific problems of bit instability are created in the instance of the bi-center bit. Bi-center bits have been used sporadically for over two decades as an alternative to undereamers. A desirable aspect to the bi-center bit is its ability to pass through a small hole and then drill a hole of a greater diameter. Problems associated with the bi-center bit, however, include those of a short life due to irregular wear patterns and excessive wear, the creation of a smaller than expected hole size and overall poor directional characteristics.
As in the instance of conventional drill bits, many solutions have been proposed to overcome the above disadvantages associated with instability and wear. One such proposed solution includes the use of penetration limiters to enhance the stability of the bi-center bit. As set forth above, penetration limiters prevent disadvantages in the reduction of the rate of penetration when a high number of limiters is used. Secondly, the geometry of a bi-center bit limits the number of positions for penetration limiters on one side of the bit. Placing more penetration limiters on one side of the bit can cause a force imbalance that makes the bit less stable.
Other proposed solutions include the use a stabilizer between the pilot and the reamer. The disadvantage of this is that it requires that the pilot bit produce a true size hole. Frequently, the pilot bit will create an oversized hole which prevents the stabilizer from contacting the hole wall or allows the bit to move laterally until the stabilizer does contact the hole wall which causes the reamer to produce an undersized hole.
As a result of these and other proposed problems, the bi-center bit has yet to realize its potential as a reliable alternative to undereaming.